A water absorbent resin is widely used as one of constituents (a main constituent) of sanitary goods (absorbent article) such as paper diapers, particularly paper diapers for aged adults, sanitary napkins, and incontinence pads, in order to absorb body fluids (urine or blood).
Such a water absorbent resin is required to have not only water absorbent performance such as a water absorption capacity (without load) (an amount of a liquid which can be absorbed and held by a water absorbent resin or water absorbent agent composition; a water absorbing capacity), a water absorption capacity under load, a water absorbent speed, and permeability, but also an additional function such as a deodorizing property, an antibacterial property, a low-coloring property, and prevention of a deterioration. So far, addition of compounds such as deodorants, antibacterial agents, aromatic substances, and coloring preventing agents to a water absorbent resin has been discussed.
For example, a water absorbent resin having a deodorizing function or antibacterial function is disclosed in Patent Literatures 1 to 29, 32, 33 etc., a water absorbent resin having an aromatic property is disclosed in Patent Literature 30 etc., and a water absorbent resin having an coloring preventing function is disclosed in Patent Literature 31.
Specifically, the aforementioned patent literatures disclose deodorants and combinations of deodorants and antibacterial agents in order to provide a water absorbent resin with a deodorizing function or antibacterial function. For example, the aforementioned patent literatures disclose a water absorbent agent composition of an essence extracted from leaves of Theaceae plants and a water absorbent resin (Patent Literature 1), a water absorbent agent composition of an essence extracted from conifers and a water absorbent resin having a specific performance (e.g. discontinuous absorption capacity under load performance) (Patent Literature 2), a water absorbent agent composition in which zeolite powder is dispersed inside a water absorbent resin (Patent Literature 3), a long-acting antibacterial deodorant containing a Japanese horse-radish extract, a mustard extract or the like, a maintaining agent (absorbent gelatinizer) for maintaining an antibacterial and deodorizing effect, and a water absorbent resin (Patent Literature 4), a deodorant and antibacterial water absorbing agent (composition) containing a compound having an antibacterial function against ammonia-producing bacteria etc. and a water absorbent resin (Patent Literature 5), and a water absorbent resin composition of an aminoacetic acid chelating agent and a water absorbent resin (Patent Literature 6).
Furthermore, as an attempt to provide absorbent articles using a water absorbent resin with a deodorizing function or an antibacterial function, the patent literatures disclose an absorbent article containing manufactured tea and a water absorbent resin (Patent Literature 7) and a disposable diaper containing a water absorbent resin containing benzalkonium chloride and/or chlorhexidine gluconate (Patent Literature 8).
Furthermore, in addition to the above water absorbent resin, as a water absorbent resin (water absorbent agent composition) provided with the deodorizing function or the antibacterial function, the patent literatures disclose a gelatinous insecticide prepared by causing a water absorbent resin to absorb a volatile monoterpene compound (Patent Literature 9), a method for producing a water absorbent resin composition in which an antibacterial membrane is formed on a surface of a water absorbent resin (Patent Literature 10), a water absorbent resin composition containing antibacterial phosphate (Patent Literatures 11 and 12), a water absorbent agent composition containing tannic acid (salt) and complex silicate compound (Patent Literature 13), a water absorbent agent composition containing a natural antibacterial ingredient extracted from grapefruit seeds and/or herbs (Patent Literature 14), a particulate water absorbent agent composition containing plant powder whose deodorizing index is not less than 180 (Patent Literature 15), a water absorbent agent composition obtained by covalent-bonding and/or ion-bonding cyclodextrin and/or cyclodextrin derivative (Patent Literature 16), a copolymer obtained by copolymerizing C2-C8 olefin or styrene and an acid anhydride (Patent Literature 17), a water absorbent resin composition containing keto acid (Patent Literature 18), a water absorbent resin composition containing thiophosphoric triamide substituted by C1-C30 alkyl group (Patent Literature 19), a water absorbent resin composition containing an urease inhibitor (Patent Literature 20), a water absorbent resin composition containing specific plants such as a ginkgo, willow, and meadow sweet (Patent Literature 21), a water absorbent resin composition containing hydrolyzable tannin (Patent Literature 22), a water absorbent resin composition containing oxidase (Patent Literature 23), a water absorbent resin composition containing 0.001 wt % of a chelating agent and 0.001-5 wt % A of tannin (Patent Literature 24), a water absorbent resin coated with triclosan (Patent Literature 25), a water absorbent resin composition containing metal oxide and metal peroxide (Patent Literature 26), a process for producing a superabsorbent, in which an anti-microbial agent and polyol are added concurrently with or immediately after surface-treating the superabsorbent (Patent Literature 27), a method for producing water-absorbing polymer particles, in which an amount of iron is controlled to be small and a chelating agent is added to a surface of a water absorbent resin (Patent Literature 28), a water absorbent resin composition in which dry-distilled essences of bamboo and green tea are blended with a water absorbent resin and which is disclosed as a water absorbent resin that is highly safe and has an antibacterial (bacteriostatic) function and a deodorizing function (Patent Literature 29), a water absorbent resin composition containing tannin whose molecular weight is not less than 1,000 (Patent Literature 32), and a water absorbent resin composition containing zinc salicylate (Patent Literature 33).
Furthermore, as a water absorbent resin having an aromatic property, the patent literatures specifically disclose a water absorbent resin containing at least one substance selected from cyclodextrin having a fragrant material, dextrin derivative, C6-C10 unsaturated aliphatic aldehyde, and C6-C10 unsaturated aliphatic alcohol and a polycarboxylic acid water absorbent resin (Patent Literature 30). Furthermore, as a water absorbent resin provided with a coloring inhibiting function, there is disclosed a water absorbent resin whose L value in a Lab color system is not more than 90 (Patent Literature 31).
Other than the water absorbent resins disclosed in the aforementioned patent literatures, there are proposed many water absorbent resins containing compounds each of which gives an additional function such as a deodorant, an antibacterial agent, an aromatic agent, a coloring preventing agent, and a deterioration inhibitor. Among them, the water absorbent resins using natural ingredients as disclosed in Patent Literatures 1, 2, 4, 7, 13-15, 21-24, 29 etc. are preferably used due to their high safeness. In the steps of producing such water absorbent resins, qualities thereof are exactly managed, and only water absorbent resins whose physical properties (water absorbent performance and the aforementioned additional function) satisfy predetermined standards (so-called acceptable products) are sold.
The inventors of the present invention have found that a water absorbent resin which contains a natural ingredient and to which an additional function, a deodorizing function in particular, is given decreases the additional function with time after the water absorbent resin is sold with its targeted quality satisfied. That is, the inventors of the present invention have found that since the additional function decreases with time after the water absorbent resin is produced, a period during which the water absorbent resin can be stored or stocked, i.e. a period during which a quality of the additional function given to the water absorbent resin is assured becomes shorter. Furthermore, the inventors of the present invention have found a problem such that in actual use of the water absorbent resin, the additional function cannot be carried out sufficiently.
On the other hand, although some documents of the conventional techniques, including the Patent Literatures 1 to 33, disclose a water absorbent resin to which an additional function (deodorizing function in particular) are given, none of those documents mentions a decrease in additional function with time, and therefore does not disclose use of a temporal deterioration preventing agent in the present invention.